In this field of technology, research and development on a next generation mobile communications system including a radio access method, retransmission control, handover and the like has been advancing at a rapid rate.
FIG. 1 shows an example of a retransmission procedure. On the right-hand side of the drawing, operations of an entity (Tx MAC) of a Medium Access Control (MAC) sub-layer and an entity (Tx RLC) of a Radio Link Control (RLC) sub-layer on the transmitting side are shown. On the left-hand side of the drawing, operations of an entity (Rx MAC) of the MAC sub-layer and an entity (Rx RLC) of the RLC sub-layer on the receiving side are shown. In a downlink, a base station is a transmitter and a user apparatus is a receiver. In an uplink, the user apparatus is the transmitter and the base station is the receiver.
In steps S21, S22, a packet to be transmitted is prepared. The MAC sub-layer requests the packet to be transmitted from the RLC sub-layer (new data request). In response to the request, the packet to be transmitted is prepared in the MAC sub-layer. In the illustrated example, a sequence number of “zero” (SN=0) is given to this packet, i.e., a packet data unit (PDU).
As shown in step S11, the packet prepared in the transmitting side is transmitted to the receiving side. In this case, the packet data unit including user data specified by the sequence number (SN) is transmitted through a data channel, and other pieces of control information such as user identification information (UE-ID), a process number (Proc), and a new data indicator (NDI) are transmitted through a control channel. A system frame number (SFN) indicating absolute transmission timing within a cell is broadcasted through a broadcast channel, and accordingly is used.
The receiver that receives the control channel and the data channel along with the broadcast channel carries out, for example, a Cyclic Redundancy Check for the received packet in order to detect an error. Accordingly, the error detection result is negative (NACK) or positive (ACK). The former shows that an error beyond an acceptable range is detected, and the latter shows the contrary. In the illustrated example, no error is detected, or the error, if detected, is within the acceptable range (CRC:OK).
As shown by step S12, the error detection result is reported to the transmitting side. When the error is detected, the negative acknowledgement is reported to the transmitting side. The transmission side in turn specifies a packet related to the negative acknowledgement and then retransmits the specified packet. The packet transmitted from the transmitting side is stored in a buffer (retransmission buffer) after the radio transmission is completed, and discarded when a positive detection result is obtained. Therefore, when the negative detection result is detected, the packet that was already transmitted is identified and retransmitted.
In the illustrated example, the positive acknowledgement (ACK) is transmitted at step S12. Because the positive acknowledgement shows that a packet has been received without an error by the receiving side, the next packet may be transmitted. However, if it is identified later that the Tx MAC falsely determines that the error detection result is positive, it is difficult to retransmit the packet for which the error detection result should have been negative. This is because the packet is discarded and window control is advanced after the positive acknowledgement is reported to the Tx RLC layer.
In order to address such a concern, after the Tx MAC determines that the error detection result is positive, a timer is activated in the related art, so that a signal for reporting false determination has been made in the error detection is deferred for a fixed period of time. The acknowledgement may be referred to as a False Ack Indicator (FAI).
Then, subsequent packet data are prepared as shown in steps S23, S24 and transmitted as shown in step S13. In the illustrated example, a packet of SN=1 is transmitted along with pieces of control information Proc=0 and NDI=1 at timing of SFN=8.
When the FAI is not transmitted, the Tx MAC confirms that a predetermined period of time has passed, as shown by “EXPIRY OF TERM” in FIG. 1. With this, there is more assurance that the error detection result is determined to be positive.
As shown in step S25, a report indicating that the error detection result is positive is made to the upper layer (Tx RLC). As a result, the packet (SN=0) subjected to the report is discarded from the transmission buffer, and window control is advanced. Such a method is described, for example, in R2-060907, “MAC functions: ARQ”, Samsung.